Botulinum neurotoxins are synthesized as single chain progenitors of mol wt 150,000. The relatively low specific toxicity of the progenitor is increased to the expected characteristically high neurotoxicity when it is activated by enzymes of the culture during toxin production or by treatment with trypsin. The fully activated toxin is a dichain molecule (mol wt 150,000) in which a heavy (mol wt 100,000) and light (mol wt 50,000) subunit chains are linked by disulfided and and isolation of botulinum enzymes that can activate progenitor toxin. Another study transition from low to high toxicity. Efforts are being made to isolate the heavy and light chains in a form that would permit study of their individual antigenicity and toxicity. Boutulinum neurotoxins exist in cultures as association complexes with a nontoxic protein. Attempts will be made to see whether these complexes are formed by nontoxic protein combining with the heavy or light chain or with both. BIBLIOGRAPHIC REFERENCES: Yang, K. H., and H. Sugiyama. 1975. Purification and properties of Clostridium botulinum type F toxin. Appl. Microbiol. 29:598-603. Sugiyama, H., S. L. Brenner and B. R. DasGupta. 1975. Detection of Clostridium botulinum toxin by local paralysis elicited with intramuscular challenge. Appl. Microbio. 30:420-423.